Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys corrode much more rapidly than pure aluminum.
Accordingly, there is a need to treat aluminum alloy substrates with pretreatments or other chemicals that provide improved corrosion resistance as well as good adhesion for polymers.
In the prior art, chemical conversion coatings have been formed on aluminum alloys by "converting" a surface of the metal into a tightly adherent coating, part of which consists of an oxidized form of aluminum. Chemical conversion coatings provide high corrosion resistance and improved adhesion for polymer coatings. A chromium-phosphate conversion coating is typically provided by contacting aluminum with an aqueous solution containing hexavalent chromium ions, phosphate ions and fluoride ions. In recent years, concerns have arisen regarding the pollution effects of chromates and phosphates discharged into waterways by such processes. Because of the high solubility and strongly oxidizing character of hexavalent chromium ions, expensive waste treatment procedures must be employed to reduce the hexavalent chromium ions to trivalent chromium ions for waste disposal.
Attempts have been made in the prior art to produce acceptable chromate-free conversion coatings for aluminum. For example, some chromate-free conversion coatings contain zirconium, titanium, hafnium and/or silicon, sometimes combined with fluorides, surfactants and polymers such as polyacrylic acid. In spite of the extensive efforts that have been made previously, there is still no entirely satisfactory non-chromate conversion coating or primer for improving the adhesion and corrosion resistance of polymer coated aluminum alloy substrates. Polymer adhesion and corrosion resistance are important characteristics in aluminum alloy sheet used for making food container bodies and ends and beverage container ends.
Attempts have also been made in the prior art to pretreat substrates with various organophosphorus compounds before coating them with a polymer. As used herein, the term "organophosphorus compounds" includes organophosphoric acids, organophosphinic acids, organophosphonic acids, as well as various salts, esters, partial salts, and partial esters of such acids. For example, Dutch Patent Application No. 263,668, filed Apr. 14, 1961, discloses a process wherein steel sheets are treated with a vinylphosphonic acid/acrylic acid copolymer before coating with an alkyd resin enamel. Although some organophosphorus pretreatments may perform adequately, they are expensive to implement. Accordingly, there still remains a need to provide an efficient and economical process for pretreating an aluminum alloy substrate with an organophosphorus compound before applying a polymer coating.
A principal objective of the present invention is to provide an efficient and economical process for pretreating an aluminum alloy substrate with an organophosphorus compound before applying a polymer coating.
To accomplish this principal objective our process provides for removing aluminum and other cations from pretreatment solutions, thereby avoiding costly disposal of such solutions.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.